I'm pretty much in the same boat. This scored me below median, but I scored in the 99th percentile on the SAT verbal as well (although that was a decade ago for me).

I think one contributing factor is the structure of the questions. If you ask me if I KNOW at least one definition for the word mawkish for example, I'll choose no I don't know a definition of it. I do however have a good enough feel for the word, that I can almost guarantee I can get a SAT style analogy with it correct, or if you gave me a multiple choice selection of definitions I can probably pick out the right one. I don't consider either of those skills equivalent to actually knowing a definition of a word.